Quantitative tissue characterization of the myocardium, with its promise for prognostic and diagnostic value in a plethora of cardiomyopathies, has generated considerable research interest and triggered a large number of clinical studies. Available tools for MR-based quantitative tissue characterization in the heart include perfusion imaging, T1-mapping techniques, T2-mapping techniques, and T2*-mapping mapping techniques, and combinations thereof.
Many of these approaches, however, are susceptible to the distribution of the radio frequency (“RF”) transmit field (“B1+”) and the resulting excitation flip angle. This problem is particularly severe when imaging at high and ultra-high magnetic field strengths, due to the increased heterogeneity of B1+. MR-based quantification accuracy greatly improves when correction for B1+ is included. Therefore, obtaining reliable absolute B1+ magnitude maps (|B1+|) is desirable for achieving accurate quantification in the presence of B1+ heterogeneity.
However, quantification of the transmit B1 fields in the heart remains challenging due to cardiac and respiratory motion, and has received limited attention. Recent studies explored cardiac B1+ mapping using the saturated double angle method (“SDAM”), where |B1+| is derived from the ratio of two images acquired at different flip angles. In SDAM, an additional saturation preparation allows shortening of the repetition time (“TR”), as waiting for full magnetization recovery is no longer required.
Breath-holding is most commonly used in these cardiac B1+ mapping methods for respiratory motion compensation. However, the acquisition of two separate images along with the use of segmented k-space readout schemes causes high sensitivity to motion. Hence, B1+ map quality may be critically impaired by residual motion, as commonly observed in patients despite breath-holding, and remains a major limiting factor for quantitative cardiac imaging.